This invention relates generally to methods of logging a well and more particularly, but not by way of limitation, to methods of detecting a fracture of an earthen formation having a proppant deposited in the fracture, which proppant at least initially includes a non-radioactive substance which is irradiated after the proppant has been safely placed into a flow system of an oil or gas well traversing the earthen formation.
During the development and operation of oil or gas wells, various types of logs are sometimes obtained for providing information about the conditions or geological structures of the downhole environment of the well. Pressure, temperature, resistivity, porosity, fracture locations, fracture types, fracture numbers, fracture thicknesses and fracturing method effectiveness are examples of information sought to be obtained from various logging techniques. These techniques have included mechanical, electrical and radiological methods. It is the radiological methods to which the present invention relates.
In performing radiological logging methods, different nuclear or atomic reactions or characteristics are detected. For example, in a known neutron-neutron logging operation, the relative neutron absorbabilities of different materials found in an earthen formation traversed by the well are used to show the distinctions between such materials and their relative locations in the formation. In neutron-gamma logging, gamma radiation is detected to show the distinctions. During such logging operations the detected characteristics are induced by an external source of neutrons that bombards naturally occurring materials. One shortcoming of these techniques is their lack of sensitivity or ability to clearly distinguish among the various radioactivities or absorbabilities of the various types of substances naturally found in the formation, which substances are not known to the personnel conducting the tests without first conducting some other type of test (for example, the taking of a core sample). Such other test would slow down the radiological logging techniques and thereby defeat one of their benefits, which is their ability to be relatively quickly performed.
Another radiological method includes injecting into the earthen formation a known radioactive substance, such as a radioactive isotope-coated propping agent, which can be readily traced as to its location within the formation. This method has been implemented with both radioactive isotope-coated particulate proppant and with radioactive liquids or solutions. A significant shortcoming of this method is the risk of exposure or contamination of personnel, equipment and the environment at the surface of the well where such radioactive substances must be handled by personnel under whatever weather conditions exist at the time. This risk of exposure or contamination also exists with respect to the well bore equipment through which the radioactive substance must flow and with respect to the formations into which the radioactive substance may uncontrollably flow. Additionally, if the radioactive proppant flows back up the well, there is the risk of a possibly uncontrollable exposure or contamination at the surface. Another disadvantage of this prior method is that because all of the injected substance is radioactive at the time it is injected, the entire injected quantity continues to radioactively decay so that additional logs cannot be run at some indeterminate time in the future after which the injected material has sufficiently decayed to preclude accurate detection. Therefore, additional injections of the radioactive substances must be performed to conduct additional logs after such time, which additional injections create further risks of exposure and contamination.
A more indirect radiological method for determining features of the downhole earthen formation, and one from which no direct downhole log is obtained, is disclosed in U.S. Pat. No. 3,002,091 to Armstrong. The Armstrong method pertains to a non-radioactive material which is flowed into one well, through a formation and out another well, at which point of production from the second well the material is irradiated at the surface to determine how much of the material made it through the formation, thereby giving some indication of the nature of the formation. A shortcoming of this technique is that it produces a radioactive substance at the surface of the second well, which radioactive substance needs to be carefully handled and disposed of to prevent exposure and contamination. Thus, this initially non-radioactive method still poses exposure and contamination risks by being subsequently made radioactive at the surface after it has flowed through the formation. Additionally, this technique does not identify specific locations of features of the earthen formation.
Despite the variety of radiological methods currently known, there is still the need for an improved method which overcomes the shortcomings of these various prior methods. Such an improved method should inject into the well an external substance which has a known nuclear or atomic reaction or characteristic which is readily distinguishable from any such reactions or characteristics of the naturally occurring substances in the formation, but which can be safely handled at the surface to overcome the significant risks posed by those prior art methods wherein radioactive substances must either be handled in putting them into the system or in extracting them from the system. Such an improved method should also produce a log directly indicating specific information as to the location of features in the formation, and such a method should allow multiple logs to be obtained over an indeterminate period of time using the original quantity of the external substance initially injected. Such a method should also be capable of being performed with known types of radiological equipment, such as that used in heretofore known neutron-neutron logging and neutron-gamma logging operations. Such a method should also be controllable as to where the external substance is to be injected and as to how much of the deposited quantity has been used to create the detectable nuclear or atomic reactions or characteristics.